J. CHEM. SOC. PERKIN TRANS. I 1992 Allene Synthesis via Boron-stabilised Alkenyl Carbanions Andrew Pelter," Keith Smith * and Kevin D. Jones Department of Chemistry, University College of Swansea, Singleton Park, Swansea SA2 8PP, UK A new allene synthesis involving the boron-Wittig reaction of aldehydes with boron-stabilised carbanions is disclosed and its scope and limitations are explored. We have previously demonstrated the versatility of the boron- Wittig reaction for the synthesis of E-and Z-alkenes,'*' erythro- 1,2-diols3 and ketones4 We have also reported on the ready production of boron-stabilised alkenyl carbanions, and ex-plored their protonation and alkylati~n.~ We have now found that boron-stabilised alkenyl carbanions undergo a facile boron-Wittig reaction with aldehydes to give allenes, according to eqn (1).i,li iii iv R1C=CSnMe3 - R'CH=amp;F4$ -R1CH=C=CHR3 ('1 1 2 Reagents: i, R2BH; ii, MesLi; iii, R3CHO; iv, TFAA. We used 1-trimethylstannyloct- 1-yne and l-trimethyl-stannyl-2-phenylethyne as typical starting materials. Our results are given in Table 1. It is clear that the carbanions derived from aryl- or alkyl ethynes behave similarly with either aromatic or aliphatic aldehydes to give allenes 2. Thus the reaction is reasonably general. Substitution of an electron-withdrawing 4-nitro group enhances the reaction (expt. 8,9) but a far longer reaction than usual is required for 4-methoxybenzaldehyde (expt. 6, 7). The presence of a chlorine or a bromine atom on the 4-position is deleterious to the reaction (expt.10-13), presumably due to metal-halogen exchange. Aliphatic aldehydes generally give allenes in acceptable yields, and are not subject to great steric inhibition (expt. 14- 22). However, the very readily enolisable phenylacetaldehyde gave low yields (expt. 23-24). In an effort to overcome the latter restriction, anhydrous salts were added to the solution of the lithio-carbanion. The best salt proved to be cadmium chloride, which strongly enhanced the yields of products from phenylacetaldehyde and led to some improvement of yield in most other cases tried (Table 1). Thus, a new and general process involving overall the condensation of alk-1-ynes with aldehydes to give 1,3-disubstituted allenes has been uncovered, and this once again illustrates the versatility of the boron-Wittig reaction.Analogies are few, one such being the four-step conversion of alkynylphosphonates into allenes in undisclosed overall yields6 Another is the condensation of 1-lithio-1-triphenylsilylethene with aldehydes, followed by chlorination and elimination to give mono-substituted allenes in moderate yields.' Typical Reaction: Preparation of 1,3-Diphenylpropa- 1,2- diene.-A freshly prepared solution of 1 -trimethylstannyl- 1- dimesitylboryl-2-phenylethene (4 mmol) in THF was cooled to -78 "C before dropwise addition of a THF solution of mesityllithium (6 mmol)* with constant stirring. The resultant dark solution was stirred at -78 "C for 30 min before addition of a solution of benzaldehyde (0.918 g, 8.55 mmol) in THF (5 cm3), followed by TFAA (1.80 g, 10mmol) in THF (5 cm3).The Table 1 Preparation of allenes from 1-trimethylstannylalkynes according to eqn (1) Yield () of 2' Expt. R' R2 R3 GC Isolated 1 Ph Mes * 72(75)' 61 2 Hex Chx 71 (76) 63 3 Hex Mes 76 65 4 Ph Mes 67 49 5 Hex Chx 69 52 6 Ph Mes 41 (52)e 22 7 Hex Chx SO(59)e 29 8 Ph Mes 74 58 9 Hex Chx 82 66 10 Ph Mes 7 (9) -11 Hex Chx 4(7) -12 Ph Mes 11 (10) -13 Hex Chx lO(6) -14 Ph Mes 64(72) 51 15 Hex Mes 74 59 16 Hex Chx 68 (75) 61 17 Ph Mes 71 57 18 Hex Chx 73 50 19 Ph Mes 75 56 20 Hex Mes 76 53 21 Ph Mes 61 46 22 Hex Chx 57 48 23 Ph Mes 15 (36) 4 24 Hex Chx 12(44) 4 ~ ~~ a All yields are for the overall process, and are based on starting alkynes 1.'Mes = mesityl (2,4,6-trimethylphenyl). Figures in parentheses are GC yields after addition of CdCI, to the lithio-anion. dChx = cyclohexyl. Yields after stirring at room temperature for 12 h. dark colour faded after ca. 10min and the reaction mixture was allowed to warm to room temperature with stirring and then stirred for a further 30 min. Addition of moist THF was fol-lowed by ether extraction and drying (MgS04). GC analysis, by comparison with a pure sample of lY3-diphenylpropa- 1,2-diene' showed that this compound was present in 72 yield. Purification was accomplished by chromatography in subdued light on neutral silica using a 1 :1 mixture of diethyl ether and light petroleum (b.p.30-40 "C). This gave pure 1,3-diphenyl- allene (0.452 g, 61) identical in all respects with a pure sample. Acknowledgements We thank the SERC for financial support of this work through a studentship to K. D. J. References 1 A. Pelter, B. Singaram and J. W. Wilson, Tetrahedron Lett., 1983,24, 635. 2 A. Pelter, D. Buss and E. Colclough, J. Chem. Soc., Chem. Commun., 1987,297. J. CHEM.SOC. PERKIN TRANS. 1 1992 3 A. Pelter, D. Buss and A. Pitchford, Tetrahedron Lett., 1985,26,5093. 8 A. Pelter, B. Singaram and J. W. Wilson, TetrahedronLett., 1983,24, 4 A. Pelter, K. Smith, S. Elgendy and M. Rowlands, TetrahedronLett., 631. 1989,30,5643. 9 A. Claesson and L.4. Olsson, J. Am. Chem. SOC.,1979,101,7302. 5 A. Pelter, K. Smith, K. D. Jones and D. E. Parry, Aust. J. Chem.,1992, 45,57. 6 M. Boran Marszak, M. Simalty and A. Seuleimon, Tetrahedron Lett., Paper 2/00367H 1974.1905. Received 23rd January 1992 7 T. HiChan and W. Mychajlowskij, TetrahedronLett., 1974, 171. Accepted 5th February 1992
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